Hi FolksI've just bought an amazingly clean 24 channel 500 series board. With the exception of the print being worn off of the channel on buttons it looks new and there's not a scratchy pot or fader to be found anywhere. Amazing!There is however a hum in the master section that I can't chase down. It's in the CR, studio and headphone feeds. When i switch the master input to any of the 2 track inputs it goes away. Switch back to mix and there it is. It's present with all of the input channels switched off and out of the mix bus. Same goes for groups and returns. They don't add or subtract from the hum.I just did a full recap on the power supply hoping that would clear it up but the recap has no effect.

Any ideas where I should look next? I've heard about people improving on the console grounding scheme but I don't think this is just normal ground hum. It's much louder than I'd expect from that.

Check the 1000 uf caps used to decouple the headphone/CR section. If they are dried up you could get hum. A scope will help find the place where it comes up. 2200 uf/35V Panasonic FR caps would be a good replacement.

Thanks Jim. Will do. Would those caps be upstream of the mix, 2tr a, 2 trc b, & 2trc c buttons? The 2 track inputs are fine, no hum. The hum is only when the mix input is assigned in the master section.